The present invention relates to reinforcing pipe and, more particularly, to a method and apparatus for reinforcing pipe in a pipeline which carries gas and/or liquid under pressure. The method and apparatus according to the present invention is especially useful in reinforcing a pipeline in service which has corroded or otherwise deteriorated over time.
Pipelines for carrying gas or liquid under pressure are ordinarily made of steel in order to with stand the fluid pressures necessary to transport fluids over large distances. Even though measures are taken during the manufacture or installation of the pipe in the pipeline to prevent corrosion, corrosion occurs. Pipelines buried in the ground are subject to deterioration from electrolytic and biochemical corrosion, cyclical soil stress, cathodic disbanding, and mechanical damage from machinery used to install the pipeline or to expose, inspect and repair the pipeline after installation. In addition, attachments to the pipe, such as valves, made of metal dissimilar to the metal of the pipe can result in galvanic corrosion, as can damp soil. Over time, the pipes exhibit pits and dents.
Currently, the most common way of reinforcing a deteriorated pipeline is to detect areas of corrosion by means of a xe2x80x9csmart pigxe2x80x9d or cathodic surveys, dig up the deteriorated regions of the pipeline, remove any corrosion protection material which may have been placed around the pipe, clean the surface by shot-blasting, and apply a primer. The pressure of fluid flow through the pipe is reduced, and split steel sleeves are welded or bolted onto the pipe. A plurality of the sleeves are installed end to end until the entire deteriorated area of the pipeline is covered. The pressure of the fluid is boosted to normal and the pipeline buried.
Drawbacks of this process are that the steel sleeves are very heavy and, therefore, require cranes and several men to move them to the pipeline and into the proper position. The welding process is time consuming and requires skilled workers. The sleeves are welded longitudinally and circumferentially. The presence of longitudinal welds prevents the sleeves from providing a gas-tight or liquid-tight seal. The circumferential welds between the sleeves and having a high carbon content create a heat effected zone in the pipe which is structurally weakened.
The sleeves are intended to take up loading from the corroded portions of the pipe. In order to transfer the load from the pipe to the sleeve, there must be a tight fit of the sleeve around the pipe. This is difficult to achieve with sleeves because the pipes tend to become distorted and, when they do, the sleeves do not fit properly and must be polished, that is, they must be ground to properly fit the pipe so as to adequately take up the load. Furthermore, there is a considerable amount of pipeline in and around oil refineries and tank farms where welding cannot be done.
The present invention is directed to a method for restoring the burst strength of a pipe having an unbreached weakened region to at least the level for which the pipe was designed. For example, by the method of the present invention, the burst strength of a pipe having an unbreached weakened region can be restored to a level at which the pipe can withstand the forces of a pressure fluid in the pipe having a pressure of at least 500 pounds per square inch.
Through the use of the method and apparatus according to the present invention, a pipeline can be reinforced without removing the pipeline from service, without using cranes or other heavy equipment for moving the reinforcement into position, and without employing welding or other time consuming procedures. the pipeline can also be reinforced without the application of heat which would cause weaknesses in the pipe and present a hazard to safety and without the need for skilled labor.
The present invention achieves these advantages through the use of bands of elongate unidirectional, lightweight, non-metallic, high tensile strength filaments in a resin matrix cured to form a strip in the shape of a coil similar to a clock spring, wherein the coil has an elastic memory. Although the coil has a continuously changing radius of curvature, the coil bands for a pipe of predetermined outer radius are chosen so that the radii of all portions of the coil band are smaller in the relaxed state of the band than the outer radius of the pipe, so that, when the bands are wound around the pipe, the elastic memory or set in the convolutions of the bands helps maintain the bands in contact against the pipe, for the first convolutions, and in contact with the underlying convolutions for the later convolutions. Because the resin matrix for the filaments is already cured at the time of installation, there are no fumes associated with the matrix during the installation of the bands. The problem is especially acute since work usually must be performed in a ditch where any fumes would accumulate. Furthermore, work often must be performed in adverse climatic conditions in which uncured resins will not work properly.
In order to reinforce an existing pipeline, the earth is dug out around and under the pipeline, as is conventionally done, and the surface of the pipe in the pipeline is prepared in a conventional manner.
The coil bands are carried manually to the pipe and manually supported and wound around the pipe, most efficiently by a two-person team. For each band, the outer end of the coil is secured to the pipe by an adhesive pad, and the band is unwound from the coil and onto the pipe as the coil is moved around the pipe. The pipe is coated with a layer of adhesive, for example, with a paint roller or brush or by spraying, and the coil is moved around the pipe. A coating of the adhesive is applied to the outer surface of each convolution of the band as the band is wound around the pipe so that a continuous layer of adhesive is defined between adjacent convolutions of the band. When the band is completely unwound from its coil and onto the pipe, it forms a new coil, inverted with respect to the original coil. The next band is brought into position and installed in the same manner, so that its edges abut the edges of the first reinforcing band. The bands are in intimate contact with the pipe, either directly or through the load transferring filler material, by which the load on the pipe is transmitted to the bands. As a result, the bands reinforce the pipe, preventing the pipe from bulging due to stresses too great for the weakened metal alone and thereby preventing failure of the pipe. Coil bands having a plurality of, typically about five, convolutions provide the deteriorated pipe with greater burst strength than it had when it was new, allowing the pipe to withstand up to twice the bursting pressure where the wall of the pipe is completely reinforced with the bands.